Differences between earlier /6 engines and newer ones

Well I have a 67 Dart that had its original slant in it. I had to switch it to a slant from 73, I only bought the stuff between the oil pan and valve cover meaning i transfered carburetors, intakes etc. from the old engine.
There is no noteable difference in performance of the two engines.
It is like the article says, the measuring methods changed and that is why hp readings differ.